I've been trying to think of a way to correalte how good at team is based on the three major facets of the game: pitching, offense, and defense.  I chose to look at ERA, Runs Scored, and Fielding %...I realize these stats have their flaws, but they're readily available and, overall, the results seem to validate common sense.  My sample teams are the 16 teams that have made it into the World Series.  Clearly, several of those teams made the World Series despite underperforming during the regular season, but again, as you'll see, the results average out those anomalies.  For each team I found their regular season rank for each of the three statistical categories previously mentioned.  So for example, the 2005 White Sox were 4th in MLB in ERA, 13th in runs scored, and 9th in Fielding %.  I looked up the rankings for all 16 of the World Series teams and found an average ranking for each of the stats.  Here are the results:

AVG. ERA Rank: 6.63

AVG. Runs Scored  Rank: 9.00

AVG. Fielding % Rank: 9.57

Normalizing the results (dividing each average by 6.63), you get

ERA= 1.00, Runs= 0.74, Fielding= 0.69

Basically, this means pitching is 26% more important than runs scored and 31% more important than fiedling %.  These results seem to validate what we already knew: pitching is the most important component of a winning team.  But also, you need to balance this with offense and defense. 

So  we can say that the Team Strengh Number is calculated by:

1.00(ERA Rank) + 0.74(Runs Scored Rank) + 0.69(Fielding % Rank)

Looking at 2008 rankings for each of the three stats (per espn.com),

the Sox's Team Strength No. is:

1.00(1) + 0.74(8) + 0.69(14) = 16.58

then Tampa Bay at 20.27, Philly at 20.64, Atlanta at 21.47, and the Cubs round ou the top 5 at 22.04.  Note that the lower the number, the higher the Team Strength

Obviously these #'s will fluctuate throughout the year, but I contend that by the time all 162 games are played, this calculation will give you a pretty good indicator of how a team stacks up against the others as well as how likely they are to succeed in the playoffs.  In addition, it allows you to compare teams from past years.  For example, the 2005 White Sox had a Team Strength Number of 19.83.  The Red Sox of 2007 were at an astounding 7.72.  We'll see how the numbers look in a couple of months.